Mixed powder for powder metallurgy

ABSTRACT

An objective of the present invention is to provide a mixed powder for powder metallurgy that makes it possible to improve mold-filling ability and reduce spread in weight of molded bodies. The mixed powder for powder metallurgy according to the present invention is obtained by mixing a graphite powder with an average particle diameter D50 of 1.0 μm or more to 3.0 μm or less and D90 of 10 μm or less, without adding a binder, with an iron-based powder, while applying a sheer force. The thus obtained mixed powder for powder metallurgy according to the present invention is characterized by including the iron-based powder and the graphite powder present so as to be collected in concave portions of the iron-based powder.

TECHNICAL FIELD

The present invention relates to a powder metallurgy technique for manufacturing a sintered body by molding and sintering a mixed powder for which an iron-based powder is a main starting material, and more particularly to a mixed powder for powder metallurgy with which packing ability to pack a mold with the mixed powder can be improved and spread in weight of the obtained molded bodies can be reduced.

BACKGROUND ART

In powder metallurgy in which a sintered body is manufactured using an iron powder or a copper powder as a main starting material, a mixed powder is usually used which includes a powder serving as the main starting material, a graphite powder for improving physical properties of a sintered body, an auxiliary starting material powder such as an alloying component, a lubricant, and the like. In particular, a carbon-supplying component, that is, a carbon source, such as graphite, is added, molding is performed, and the carbon source is then caused to diffuse and penetrate into an iron powder in the heating and sintering step in order to improve mechanical properties, such as strength and hardness, of the sintered body.

However, the specific gravity and particle diameter of graphite are less than those of the iron powder, and the resulting problem is that where graphite is simply mixed with the iron powder, the two are significantly separated, the graphite segregates, and uniform mixing is impossible. In powder metallurgy, a mixed powder is usually stored in advance in a storage hopper to enable mass production of sintered bodies. In the storage hopper, graphite with a small specific gravity tends to segregate to the upper layer portion in the hopper, and when the mixed powder is discharged from the hopper, the concentration of graphite becomes higher in the latest part of the discharge from the hopper, so a cementite structure precipitates in the portion of the sintered body with a high graphite concentration, resulting in degradation in mechanical properties. Where spread in the amount of carbon in the sintered body occurs due to graphite segregation, products with stable quality are difficult to manufacture. Further, the segregation of graphite causes dusting of the graphite powder in the mixing step or molding step, and this worsens the working environment and makes the mixed powder more difficult to handle. This segregation occurs not only in graphite but also in a variety of other powders which are mixed with iron powders, and there is a demand to prevent such segregation.

Generally, the following three methods have been suggested for preventing the segregation and dusting. In the first method, as disclosed, for example, in Patent Literature 1 and 2, a liquid additive such as tall oil is added to a mixed powder. The merit of this method is that the mixed powder can be manufactured with simple equipment, but a problem lies in that when the liquid additive is added in an amount such that the segregation-preventing effect can be confirmed, a liquid bridging force acts between the iron powder particles and the flowing ability is greatly degraded. In the second method, as disclosed, for example, in Patent Literature 3 and 4, a solid binder such as a high-molecular polymer is dissolved in a solvent and uniformly mixed, and the solvent is thereafter evaporated to cause adhesion of graphite to the surface of an iron powder. The merits of this method are that graphite can be reliably attached to the iron powder and a wide variety of lubricants can be selected for use, but the problem associated with this method is that, although depending on the amount and type of the mixed powder, the flowing ability thereof can be insufficient or compressibility can be degraded. The third method, which is disclosed, for example, in Patent Literature 5, is the so-called hot-melt method in which a lubricant with a comparatively low molecular weight, such as a fatty acid, is heated and melted during mixing with an iron powder. In this method, in order to attach the melted lubricant fixedly and uniformly to the iron powder surface, the temperature has to be controlled throughout the mixing process and the number of lubricants that can be selected for use is limited, which constitutes a drawback. The third method is also problematic in terms of productivity, because it is necessary to wait until the lubricant cools down.

Patent Literature 6 filed by the applicant of the present application discloses a technique in which, in contrast with the three methods above, graphite with a controlled average particle diameter is mixed, without adding a binder, with an iron-based powder while applying a sheer force, thereby suppressing the segregation of the graphite powder. In addition, it is indicated that this technique also ensures excellent flowability of the mixed powder. In powder metallurgy, flowing ability is one of important characteristics of a mixed powder when the mixed powder is discharged from a storage hopper and packed into a mold. While Patent Literature 6 adopts fluidity of a mixed powder, stipulated by JIS Z2502 etc., as the indicator of flowability, mold-filling ability is also an important characteristic in addition to the fluidity, which is characterized by a mixed powder being discharged from a hopper through a hose and packed satisfactorily into a mold. When this mold-filling ability is decreased, it results in spread in weight of molded bodies.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Publication No.     S60-502158 -   Patent Literature 2: Japanese Patent Application Publication No.     H6-49503 -   Patent Literature 3: Japanese Patent Application Publication No.     H5-86403 -   Patent Literature 4: Japanese Patent Application Publication No.     H7-173503 -   Patent Literature 5: Japanese Patent Application Publication No.     H1-219101 -   Patent Literature 6: Japanese Patent Application Publication No.     2012-102355

SUMMARY OF INVENTION

It is an objective of the present invention to provide a mixed powder for powder metallurgy which has improved mold-filling ability and can reduce the spread in weight of the molded bodies.

The mixed powder for powder metallurgy according to the present invention that attains the objective is obtained by mixing a graphite powder with an average particle diameter D50 of 1.0 μm or more to 3.0 μm or less and D90 of 10 μm or less, without adding a binder, with an iron-based powder, while applying a sheer force. The mixed powder for powder metallurgy of the present invention which is thus obtained is characterized by including the iron-based powder and the graphite powder which is present so as to be collected in concave portions of the iron-based powder.

In the present invention, an average particle diameter D50 of the graphite powder is preferably 1.6 μm or more to 2.7 μm or less, and the iron-based powder is preferably an atomized iron powder or a reduced iron powder.

In accordance with the present invention, since the graphite powder with D50 and D90 within the predetermined ranges is mixed, without adding a binder, with the iron-based powder, while applying a sheet force, the graphite powder is present so as to be collected in concave portions of the iron-based powder, satisfactory mold-filling ability can be ensured, and spread in weight of the molded bodies can be reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 are schematic diagrams illustrating a mold-filling ability evaluation device used in the below-described examples, where FIG. 1A is a front view, and FIGS. 1B to 1D are cross-sectional views illustrating the states during operation.

FIG. 2 is a scanning electron microscope photograph obtained when observing the mixed powder of the present invention in the below-described examples.

FIG. 3 is a scanning electron microscope photograph obtained when observing the mixed powder of the present invention in the below-described examples.

DESCRIPTION OF EMBODIMENTS

On the basis of the technique disclosed in Patent Literature 6, the inventors have investigated the relationship between the particle diameter of graphite and spread in weight of molded bodies with the object of reducing the spread in weight of molded bodies which was not addressed in Patent Literature 6, and have conducted the following experiments. Commercial natural graphite (manufactured by Nippon Kokuen Group, CPB, average particle diameter 22.6 μm) was pulverized in a dry jet mill to obtain an average particle diameter D50 shown in Table 1. A mixed powder was obtained by mixing the graphite powder obtained by the pulverization, an iron powder (manufactured by Kobe Steel, Ltd., Atomel 300M, particle diameter: 180 μm or less, average particle diameter: 70 μm), a copper powder (manufactured by Fukuda Metal Foil & Powder Co., Ltd., CuAtw-250), and zinc stearate (manufactured by Adeka Corporation, ZNS-730) as a lubricant. The mixing ratio is 0.8 part by mass of the graphite powder, 2 parts by mass of the copper powder, and 0.75 part by mass of the lubricant per 97.2 parts by mass of the iron powder. The mixing was performed for 4 min at 300 rpm by using a high-speed mixer having a stirring blade.

A total of 300 testpieces with a target weight of 51 g were continuously molded with a mechanical powder molding press by using the obtained mixed powder, and the spread in weight of the obtained molded bodies was evaluated. The spread in weight was evaluated by the difference R (g) between the maximum weight and minimum weight of 300 molded bodies. The results are shown in Table 1.

TABLE 1 Average particle diameter of Spread graphite powder D50 (μm) in weight R (g) Reference Example 1 Graphite is not added 1.35 Reference Example 2 22.6 7.84 Reference Example 3 19.9 7.78 Reference Example 4 9.3 4.36 Reference Example 5 8.2 4.00 Reference Example 6 6.7 3.24 Reference Example 7 5.1 2.42 Reference Example 8 5.0 2.46 Reference Example 9 3.1 2.33 Reference Example 10 2.7 2.45 Reference Example 11 1.8 2.43

The spread in weight in Reference Example 1 in which no graphite powder was added during mixing was 1.35 g, and it is clear that the spread in weight in Reference Examples 2 to 11 in which the graphite powder was added was larger than that in Reference Example 1. Further, although the spread in weight generally tends to decrease with the decrease in the average particle diameter D50 of the graphite powder, it is impossible to conclude that the spread in weight is simply affected only by the average particle diameter D50 of the graphite powder, in particular, as shown by Reference Examples 6 to 11. Further, the target of the spread being about 4% or less with respect to the target weight, that is, about 2 g or less, cannot be attained by only controlling the average particle diameter of the graphite powder.

Accordingly, the inventors have conceived of adjusting not only the average diameter D50 of the graphite powder, as suggested in Patent Literature 6, but also D90. As indicated by the below-described examples, by mixing the graphite powder with adjusted D50 and D90 with an iron-based powder, while applying a sheer force, it is possible to rub the graphite powder into the concave portions present on the surface of the iron-based powder, increase the mold-filling ability, and reduce the spread in weight of the molded bodies.

To obtain the sufficient rubbing effect into the concave portions, the D50 of the graphite powder is set to 3.0 μm or less. The D50 is preferably 2.7 μm or less, and more preferably 2.5 μm or less. From the standpoint of the rubbing effect, a smaller D50 of the graphite powder is preferred, but where the D50 becomes too small, although the spread in weight can be reduced, the molded body density during press molding is greatly reduced and the strength of the part obtained by sintering the molded body cannot be ensured. Accordingly, the D50 of the graphite powder was set to 1.0 μm or more. The D50 of the graphite powder is preferably 1.1 μm or more, and more preferably 1.6 μm or more. From the standpoint of attaining all of the reduction in the spread in weight of molded bodies, the improvement of mold-filling ability, and the increase in the molded body density at a high level, it is preferred that the D50 of the graphite powder be 1.6 μm or more to 2.7 μm or less. The decrease in molded body density observed when the D50 of the graphite powder is less than 1.0 μm can be explained by the collapse of the layered structure of graphite and loss of lubricating ability of graphite due to excessive pulverization.

The graphite powder of the present invention with the D50 adjusted to the ranges mentioned hereinabove can be obtained by pulverizing the commercial natural graphite or artificial graphite, and the usual pulverizer may be used for the pulverization. The pulverization atmosphere is not particularly limited, and the pulverization may be performed with a dry or wet system. The usual pulverizer may be used, examples thereof including a roll crusher, a cutter mill, a rotary crusher, a hammer crusher, a jet mill, a vibration mill, a pin mill, a wing mill, a ball mill, and a planetary mill.

In the pulverized graphite powder, the specific surface area is increased and chemical forces are believed to be acting in addition to physical forces such as electrostatic forces. Thus, a large number of functional groups such as hydrogen groups are apparently present on the pulverized surface of the finely pulverized graphite, intermolecular forces act between the iron powder and the graphite powder through the functional groups, and the adhesion between the graphite powder and the iron powder increases. The presence/absence of the functional groups and the amount thereof can be determined, to a certain degree, by heating the graphite powder in a nitrogen atmosphere and measuring the weight variation rate from room temperature to 950° C. The temperature increase rate when the temperature is raised from the room temperature to 950° C. may be about 10° C./min. The type of gas generated from a graphite powder usually differs depending on a heating temperature range, and the type of functional groups removed in each temperature range can be estimated from the type of the generated gas. It is generally known that carboxyl groups (—COOH) or hydroxyl groups (—OH) are removed at 150° C. to 500° C., oxo groups (═O) are removed at 500° C. to 900° C., and hydrogen groups (—H) are removed at 900° C. or more. By examining the weight loss from 150° C. to 950° C., it is possible to eliminate the influence of weight loss of moisture that can be removed at a temperature lower than 150° C. and determine the type and amount of functional groups contained in the graphite powder.

In the present invention, it is important not only to adjust the D50 of the graphite powder to the predetermined range, but also to set the D90 to 10 μm or less. By setting the D90 to 10 μm or less, it is possible to reduce the amount of the graphite powder rubbed into the concave portions of the iron-based powder. The D90 of the graphite powder is preferably 9.5 μm or less, more preferably 9.0 μm or less, and particularly preferably 8.5 μm or less. A smaller D90 of the graphite powder is preferred, but the lower limit thereof is usually about 3.5 μm. In order to set the D90 of the graphite powder within such ranges, air-jet classification may be performed after pulverization in the above pulverizer.

The D50 and D90 of the graphite powder each can be measured with a particle size distribution measuring device of a laser diffraction type. The D50 means a cumulative particle diameter corresponding to 50% (based on the volume standard), and D90 means a cumulative particle diameter corresponding to 90% (based on the volume standard).

The amount of the graphite powder is usually 0.1 part by mass to 2.5 parts by mass per a total of 100 parts by mass of the iron-based powder, graphite powder, and the below-described strength enhancer. In application to parts for mechanical structures, a compounding ratio of 0.2 part by mass to 1.2 parts by mass is often used, and the graphite powder can be advantageously used in this range.

In order to attain the satisfactory mold-filling ability, it is important to mix the graphite powder and the iron-based powder while applying a sheer force, without adding a binder. As a result of applying a sheer force, it is possible to rub the graphite powder into the concave portions of the iron-based powder. Further, since no binder is added, the adhesion of the graphite powder outside the concave portions, for example, to convex portions, of the iron-based powder can be suppressed, and the graphite powder is present so as to be collected in the concave portions of the iron-based powder. Where a large amount of the graphite powder is present outside the concave portions of the iron-based powder, the flowability of the mixed powder is worsened and the mold-filling ability is degraded. With a method in which a binder is added or a method different from the below-described mixing method in which a sheer force is applied, a large amount of the graphite powder is also present outside the concave portions of the iron-based powder and satisfactory mold-filling ability cannot be attained.

Another advantageous effect of not adding a binder is that the density of the molded body molded under the same molding pressure, and the density of the sintered body obtained by sintering the molded body are higher than those obtained when a binder is added, and a sintered body with a satisfactory strength is obtained. Furthermore, with the mixed powder of the present invention to which no binder is added, a step of binder removal which is performed between a molding step and a sintering step can be omitted or simplified, which makes a contribution to the increase in productivity of sintered parts and improvement of environment.

A mixing method in which a sheer force is applied is a method different from counterflow mixing methods represented by methods implemented with a V-shaped mixer and a double-cone mixer, and mixing methods using a vibration mill or a ball mill, such as a vibro mill and an electromagnetic mill. Mixing in which a sheer force is applied can be realized, for example, by using a mixer equipped with a stirring blade. The stirring blade preferably moves so as to cut the powder, and examples of the shape thereof include paddle, turbine, ribbon, screw, multistage blade, anchor-type, horseshoe-type, and gate-type shapes. The mixer container may be of a fixed or rotating configuration, provided that it is equipped with a stirring blade. Specific examples of the mixers equipped with a stirring blade include a high-speed mixer, a plow-type mixer, and a Nauta mixer. The mixing time varies depending on the type of the mixer used and the amount of mixed powder, but is generally 1 min to 20 min.

The graphite powder and iron-based powder may be mixed by a dry or wet system. The mixing sequence of the graphite powder and iron-based powder is not particularly limited, and the powders may be simultaneously placed into the mixer, or initially one powder may be loaded into the mixer and then the other powder may be loaded. The graphite powder and iron-based powder may be mixed, for example, at a normal temperature, rather than under heating to a temperature equal to or higher than that at which a lubricant, or the like, melts, as in the so-called hot-melt method.

The powder for powder metallurgy of the present invention may include a lubricant and, for example, at least one physical property improving agent from among a strength enhancer, an abrasion resistance improving agent, and a machinability-improving agent, in addition to the graphite powder and iron-based powder. These components may be added when mixing the graphite powder and iron-based powder. The addition sequence thereof is not particularly limited and, for example, the lubricant and machinability-improving agent may be added to the mixer simultaneously with the graphite powder and iron-based powder, or the graphite powder and iron-based powder may be mixed and then the lubricant and machinability-improving agent may be added by one or in a combination of two or more thereof, while mixing, e.g., by actuating a stirring blade.

Examples of the lubricant include metallic soaps, alkylene bis-fatty acid amides, and fatty acids, and these may be used individually or in combinations of two or more thereof. A fatty acid salt can be used as the metallic soap. For example, a fatty acid salt with a carbon number of 12 or more, in particular, zinc stearate, can be advantageously used. Specific examples of the alkylene bis-fatty acid amides include C₂₋₆ alkylene bis-C₁₂₋₂₄ carboxylic acid amides, and ethylene bis-stearyl amide can be advantageously used. Compounds represented by R₁COOH, with R₁ being, for example, a hydrocarbon group, can be used as the fatty acid, carboxylic acids with a carbon number of about 16-22 are preferred, and stearic acid and oleic acid are particularly preferred. The amount of the lubricant is, for example, 0.3 part by mass or more to 1.5 parts by mass or less, preferably 0.5 part by mass or more to 1.0 part by mass or less per a total of 100 parts by mass of the iron-based powder, graphite powder, and strength enhancer.

Examples of strength enhancers include powders containing at least one of copper, nickel, chromium, molybdenum, manganese, and silicon, specific examples including a copper powder, a nickel powder, a chromium-containing powder, a molybdenum powder, a manganese-containing powder, and a silicon-containing powder. The strength enhancers may be used individually or in combinations of two or more thereof. The amount added of the strength enhancer is, for example, 0.2 part by mass or more to 5 parts by mass or less, more preferably 0.3 part by mass or more to 3 parts by mass or less per a total of 100 parts by mass of the iron-based powder, graphite powder, and strength enhancer.

Examples of abrasion resistance improving agents include hard particles such as carbides, silicides, and nitrides which may be used individually or in combinations of two or more thereof.

Examples of machinability-improving agents include manganese sulfide, talc, and calcium fluoride which may be used individually or in combinations of two or more thereof.

The iron-based powder used in the present invention may be a pure iron powder or an iron alloy powder. The iron alloy powder may be a partially alloyed powder in which an alloying powder, for example, of copper, nickel, chromium, and molybdenum is diffusion adhered to the surface of an iron-based powder, or a pre-alloyed powder obtained from molten iron or molten steel including an alloying component similar to the alloying powder. The iron-based powder may be an atomized iron powder obtained by atomizing molten iron or steel, or may be a reduced iron powder obtained by reducing an iron ore or mill scale. An iron powder which is usually used for machinery parts can be used as the iron-based powder. More specifically, an iron-based powder with an average particle diameter D50 of 70 μm to 100 μm and a maximum particle diameter of 250 μm or less, preferably 180 μm or less, is preferred. The average particle diameter of the iron-based powder means a cumulative 50% pass particle diameter when the particle size distribution is measured according to Japan Powder Metallurgy Association Standard JPMA P02-1992 (Sieve Analysis and Test Methods for Metal Powders).

In accordance with the present invention, the mold-filling ability can be increased and the spread in weight of molded bodies can be reduced. As a result of using the mixed powder of the present invention, the spread in weight estimated by the maximum value and minimum value of the molded body weight when a plurality of molded bodies is molded can be made 4% or less with respect to the target weight.

The present application claims priority based on Japanese Patent Application No. 2014-111418 filed on May 29, 2014. The entire contents of Japanese Patent Application No. 2014-111418 filed on May 29, 2014 are incorporated herein by reference.

EXAMPLES

The present invention will be explained hereinbelow in greater detail with reference to examples. The present invention is not intended to be limited to the below-described examples and obviously can be implemented with appropriate modifications within the scope adaptable to the essence described hereinbefore and hereinafter, and all those modifications are included in the technical scope of the present invention.

Commercial natural graphite (manufactured by Nippon Kokuen Group, CPB, average particle diameter 22.6 μm) was pulverized in a dry jet mill such as to obtain an average particle diameter D50 shown in Tables 4 to 6 below, and the D90 was that of the as-pulverized powder or was adjusted by air jet classification. A mixed powder was obtained by mixing the graphite powder obtained by the pulverization, an iron powder (manufactured by Kobe Steel, Ltd., Atomel 300M, 300NH, or 250M), a copper powder (manufactured by Fukuda Metal Foil & Powder Co., Ltd., CuAtw-250), and zinc stearate (manufactured by Adeka Corporation, ZNS-730) as a lubricant. The mixing ratio was 0.8 part by mass of the graphite powder, 2 parts by mass of the copper powder, and 0.75 part by mass of the lubricant per 97.2 parts by mass of the iron powder. The mixing was performed for 4 min at 300 rpm by using a high-speed mixer having a stirring blade. The following evaluations (1) to (3) were performed using the obtained mixed powder.

(1) Measurement of Spread in Weight of Molded Bodies

A total of 300 of ring-shaped testpieces with a target weight of 51 g, an outer diameter of 30 mm, and an inner diameter of 10 mm were molded by a mechanical powder molding press, and the spread in weight of the obtained molded bodies was evaluated. The spread in weight was evaluated by the difference R (g) between the maximum weight and minimum weight of the 300 molded bodies.

(2) Measurement of Mold-Filling Ability

The mold-filling ability was evaluated using the evaluation device depicted in FIGS. 1. FIGS. 1 depict a device for evaluating the mold-filling ability of a powder, the device being configured of a base 1 accommodating a cavity container 3, an air cylinder 5 fixedly provided on the base on the side other than that of the cavity container 3, and a powder supply box 2 mounted on the distal end of a rod 4 of the air cylinder 5. The powder supply box 2 is a bottomless box that, by means of an operation of the air cylinder 5, moves on an upper surface of the base 1 in a substantially air-tight manner such as to reciprocate over the cavity container 3. The cavity container 3 has a slit-shaped cavity with a width of several millimeters that is formed to extend in the direction perpendicular to the reciprocating movement direction of the powder supply box 2. FIG. 1A is a front view of the evaluation device, and FIGS. 1B to 1D are cross-sectional views illustrating the state of the powder supply box as it moves.

The measurement sequence is described below. Initially, as depicted in FIG. 1B, a predetermined amount of the powder is loaded into the powder supply box 2 in a state in which the rod 4 of the air cylinder 5 is extended. Then, the rod 4 of the air cylinder 5 is contracted, and the powder supply box 2 is allowed to pass at a predetermined rate above the slit-shaped cavity of the cavity container 3. As a result of such passing, the powder contained in the powder supply box 2 falls down into the cavity container 3, as depicted in FIG. 1C. After the powder supply box 2 passes, as depicted in FIG. 1D, the interior of the cavity container 3 is packed with the powder. The size of the powder supply box 2 is 80×80×70 mm, the size of the cavity container 3 is 80×60×55 mm, the size of the slit is 2×60 mm, and the shoe rate, that is, the passage rate of the powder supply box 2, is 100 mm/s. Three tests were performed for each Experiment No., the amount (mg) of the powder packed in each test was divided by 120 mm² which was the surface area of the slit-shaped cavity, and the average value of the obtained values was taken as the mold-filling ability (mg/mm²) of each No.

(3) Measurement of Molded Body Density

The obtained mixed powder was loaded in a predetermined mold and molded under a press pressure of 490 MPa and 686 MPa to fabricate tablet-shaped testpiece with φ11.28 mm. The density of the resultant molded body was then measured.

Properties of Atomel 300M, 300NH, and 250M, which were the iron-based powders used, are presented in Tables 2 and 3. The apparent density presented in Tables 2 and 3 was measured by the method according to JIS Z2504 (Metal Powders—Apparent Density Test Methods), and the fluidity was measured by the method according to J1S Z2502 (Fluidity Test Methods for Metal Powders). The apparent density of Atomel 300NH is large and the apparent density of 250M is small, with 300M being taken as a reference. Thus, it can be said that in 300NH, the unevenness of the iron powder surface is small and the degree of difference in shape is low, and in 250M, the unevenness is large and the degree of difference in shape is high, with 300M being taken as a reference. The apparent density of 250M is substantially the same as the apparent density of the reduced iron powder.

The results of (1) and (3) above are presented in Tables 4 to 6. Atomel 300M (average particle diameter: about 70 μm), Atomel 300NH (apparent density 3.10 g/cm³, average particle diameter: about 90 μm), and Atomel 250M (apparent density 2.42 g/cm³, average particle diameter: about 85 μm) were used as the iron-based powder in Tables 4, 5, and 6, respectively.

TABLE 2 Powder properties Apparent density Fluidity Chemical component composition (mass %) Steel grade (g/cm³) (s/50 g) C Si Mn P S O 300M 2.85 to 3.05 ≦30 ≦0.02 ≦0.05 0.10 to 0.30 ≦0.020 ≦0.020 ≦0.25 300NH 2.95 to 3.10 ≦30 ≦0.01 ≦0.03 ≦0.10 ≦0.010 ≦0.010 ≦0.20 250M 2.40 to 2.60 20 to 30 ≦0.02 ≦0.05 0.10 to 0.30 ≦0.020 ≦0.020 ≦0.25

TABLE 3 Particle size distribution (%) Steel 250 μm or 180 μm to 150 μm to 106 μm to grade more 250 μm 180 μm 150 μm 75 μm to 106 μm 63 μm to 75 μm 45 μm to 63 μm 45 μm or less 300M — ≦1 ≦10 10 to 25 15 to 30 5 to 20 8 to 23 20 to 40 300NH ≦1 ≦15 ≦15 10 to 30 15 to 30 5 to 20 8 to 23 10 to 30 250M ≦1 ≦10 ≦10 10 to 25 20 to 35 5 to 20 8 to 23 15 to 35

TABLE 4 Particle Molded diameter of body density graphite powder (g/cm³) Mold-filling Experiment D50 490 ability No. (μm) D90 (μm) R (g) MPa 686 MPa (mg/mm²) 1-1 22.6 49.8 7.84 6.81 7.15 28.5 1-2 19.9 44.6 7.78 6.81 7.15 28.9 1-3 9.3 18.3 4.36 6.82 7.16 29.9 1-4 8.2 16.4 4.00 6.82 7.16 30.6 1-5 6.7 11.8 3.24 6.83 7.16 31.5 1-6 5.1 8.9 2.42 6.84 7.17 32.6 1-7 5.0 12.1 2.46 6.84 7.17 32.0 1-8 3.1 12.3 2.33 6.83 7.17 33.8 1-9 3.0 9.8 2.00 6.83 7.16 34.5 1-10 2.7 11.8 2.45 6.82 7.16 33.5 1-11 2.7 7.8 1.99 6.82 7.16 36.3 1-12 2.5 4.7 1.97 6.81 7.16 36.6 1-13 1.8 11.7 2.43 6.81 7.16 35.9 1-14 1.7 7.0 1.98 6.81 7.16 37.2 1-15 1.6 4.2 1.96 6.81 7.14 37.0 1-16 1.1 3.9 1.95 6.80 7.11 35.5 1-17 0.8 3.7 1.90 6.77 7.08 33.7 *Atomel 300M was used as the iron-based powder

TABLE 5 Particle Molded diameter of body density graphite powder (g/cm³) Mold-filling Experiment D50 490 ability No. (μm) D90 (μm) R (g) MPa 686 MPa (mg/mm²) 2-1 19.9 44.6 7.70 6.93 7.22 29.8 2-2 5.0 12.1 2.36 6.96 7.25 33.1 2-3 2.7 7.8 1.88 6.94 7.23 37.5 2-4 1.7 7.0 1.90 6.93 7.23 37.4 2-5 0.8 3.7 1.92 6.87 7.15 34.7 *Atomel 300NH was used as the iron-based powder

TABLE 6 Particle Molded diameter of body density graphite powder (g/cm³) Mold-filling Experiment D50 490 ability No. (μm) D90 (μm) R (g) MPa 686 MPa (mg/mm²) 3-1 19.9 44.6 6.50 6.80 7.14 30.1 3-2 5.0 12.1 2.43 6.83 7.16 32.5 3-3 2.7 7.8 1.95 6.82 7.14 36.8 3-4 1.7 7.0 1.88 6.80 7.13 35.9 3-5 0.8 3.7 1.94 6.75 7.05 34.0 *Atomel 250M was used as the iron-based powder

Tables 4 to 6 indicate that in Experiment No. 1-9, 1-11, 1-12, 1-14 to 1-16, 2-3, 2-4, 3-3, and 3-4 of the present invention in which the D50 of the graphite powder was 3.0 μm or less and the D90 was 10 μm or less, the spread in weight R could be made 2.0 g or less, that is, 4% or less of the target weight, and the molded body density was satisfactory. FIG. 2 is a scanning electron microscope photograph obtained when observing Experiment No. 2-3. It is clear from FIG. 2 that the graphite powder is present so as to be collected in concave portions of the iron powder. FIG. 3 is a scanning electron microscope photograph obtained when observing Experiment No. 3-3. In FIG. 3, the state in which the graphite powder is present so as to be collected in concave portions can be also observed.

Comparing No. 1-9, 1-11, 1-12, 1-14 to 1-16 that are the invention examples in Table 4 which used Atomel 300M with No. 2-3 and 2-4 that are the invention examples in Table 5 which used Atomel 300NH, the spread in weight R in the invention examples in Table 5 is further reduced. As mentioned hereinabove, in Atomel 300NH, the unevenness of the iron powder surface is less and the degree of difference in shape is lower than those of Atomel 300M, but since the ratio of particles with a large particle diameter was high and the width of the concave portions was large, the graphite powder could be sufficiently introduced therein.

Meanwhile, in Experiment No. 1-1 to 1-8, 1-10, and 1-13, Experiment No. 2-1 and 2-2, and Experiment No. 3-1 and 3-2 in which the D50 of the graphite powder was greater than 3.0 μm or the D90 was larger than 10 μm, the spread in weight R increased. Further, in Experiment No. 1-17, 2-5, and 3-5, the D50 was less than 1.0 μm. As a result, although the spread in weight R could be made 2.0 g or less, the molded body density was less than that of the experiment examples of the present invention. Since the molded body density is also influenced by the shape of the iron-based powder, it is appropriate to evaluate the molded body density for each type of the iron-based powder. Thus, the evaluation shows that the molded body density of No. 1-17 is lower than that of No. 1-9, 1-11, 1-12, and 1-14 to 1-16, the molded body density of No. 2-5 is lower than that of No. 2-3 and 2-4, and the molded body density of No. 3-5 is lower than that of No. 3-3 and 3-4.

INDUSTRIAL APPLICABILITY

In the present invention, quality can be stabilized, for example, dimensional changes caused by the refinement of graphite powder can be minimized, and the reduction of energy consumption and cost, such as decrease in sintering temperature and shortening of sintering time, can be realized in the production of sintered parts. The mixed powder of the present invention is suitable for sintered parts for mechanical structures, in particular for thin parts and parts of complex shape. Further, since weight reduction is enabled, the mixed powder is also advantageous for high-strength materials.

REFERENCE SIGNS LIST

-   1 Base -   2 Powder supply box -   3 Cavity container -   4 Rod -   5 Air cylinder 

1. A mixed powder, obtained by a process comprising mixing a graphite powder with an average particle diameter D50 of 1.0 μm or more to 3.0 μm or less and D90 of 10 μm or less and an iron-based powder without adding a binder while applying a sheer force.
 2. A mixed powder, comprising an iron-based powder and a graphite powder present so as to be collected in concave portions of the iron-based powder.
 3. The mixed powder according to claim 1, wherein the average particle diameter D50 of the graphite powder is 1.6 μm or more to 2.7 μm or less.
 4. The mixed powder according to claim 1, wherein the iron-based powder is an atomized iron powder or reduced iron powder.
 5. The mixed powder according to claim 2, wherein the iron-based powder is an atomized iron powder or reduced iron powder.
 6. The mixed powder according to claim 3, wherein the iron-based powder is an atomized iron powder or reduced iron powder. 